1. Field of the Invention
The present invention relates to image forming apparatuses using electrophotography, such as copying machines, printers, facsimile machines and hybrid machines thereof.
2. Description of the Related Art
Mono-component development and two-component development are conventionally known examples of developing technology employed in electrophotographic image forming apparatuses using dry toner particles.
A mono-component development system is suited for high-quality imaging. This is because a mono-component developer does not contain carrier beads and, thus, an electrostatic latent image formed on a photoreceptor is not disturbed by a magnetic brush produced by a combination of carrier beads and toner particles. It is however difficult to maintain a stable amount of electrostatic toner charge in the mono-component development system. Additionally, color toner particles should necessarily be nonmagnetic as the color toner particles are required to have light transmitting properties. For this reason, a full-color image forming apparatus usually employs a two-component developing system using developers containing carrier beads which serve as a medium for charging and carrying toner particles.
An image forming method based on the two-component developing system employs so-called touchdown development (also known as hybrid development) in which a magnetic brush formed on a developer carrying member (magnetic roller) carrying a two-component developer creates a thin toner layer on a toner carrying member (development roller) and part of this thin toner layer is transferred to a latent image carrying member (e.g., a photosensitive drum) to develop an electrostatic latent image formed thereon. This method of development however has a problem that there is a difference between a proper amount of electrostatic toner charge at the time of developing the electrostatic latent image and a proper amount of electrostatic toner charge at the time of forming the thin toner layer. Therefore, the two-component image forming method is associated with such problems as low image density due to an insufficient amount of toner particles in the thin toner layer and a development ghost caused by inadequate removal of that portion of the thin toner layer which is left unused for development on the development roller.
One factor causing the aforementioned problems would be toner scattering which can occur chiefly within a developing device in a process of stirring the toner particles in a housing or in the vicinity of a magnetic roller, for example. The toner particles scattered in the developing device spread inside the electrophotographic apparatus in which a photosensitive drum, an optical system, a charging device, an image transfer device and so on are disposed, thus causing various kinds of image forming failures and malfunctions including the aforementioned problems.
In an attempt to overcome such problems of the prior art, Japanese Unexamined Patent Publication No. 1996-137256 proposes an arrangement for preventing toner scattering by using a scattering prevention member and scraping means (blade). The scattering prevention member is rotatably mounted face to face with a photosensitive drum with a narrow gap therebetween whereby a developer which is dispersed when supplied attaches to a surface of the scattering prevention member, thus preventing developer particles from scattering to the exterior of a developing device. The scraping means scrapes off the developer particles adhering to the scattering prevention member.
On the other hand, Japanese Unexamined Patent Publication No. 2005-242194 proposes an arrangement for a two-component type developing device. This arrangement includes a toner collecting roller provided in an opening of a housing of the developing device for collecting scattered toner particles. The collected toner particles are scraped off the toner collecting roller and returned to the developing device.
According to the arrangement of Japanese Unexamined Patent Publication No. 1996-137256, however, the developer particles scraped off the scattering prevention member are subjected to stress due to mechanical contact with the blade and this stress accelerates deterioration of the developer. Particularly in touchdown development, the developer is susceptible to the influence of selective development. Specifically, the stress caused by the scraping with the blade can cause external additive particles to be separated from or buried in toner particles. This would cause a change in charging characteristics of the toner particles. When the toner particles with modified charging characteristics returns to a two-component developer storage space, toner scattering and selective development would be accelerated and a reduction in image density would result, making it difficult to ensure stable image forming operation for a long period of time.
Since toner particles left unused for development on a development roller are collected by a magnetic brush in touchdown development, the collected toner particles have low adhesion to carrier beads compared to those used for ordinary two-component development. In addition, since toner concentration in the two-component developer used in touchdown development is made higher than that for the ordinary two-component developing system, the two-component developer for touchdown development has low fluidity. Therefore, during a process of toner collection, the developer is pushed in and compressed and, at the same time, surrounding air masses can find no way to go but to escape to the exterior of the developing device together with entrained toner particles, so that toner scattering is more likely to occur in touchdown development systems.
According to the arrangement of Japanese Unexamined Patent Publication No. 2005-242194, on the other hand, it is necessary to provide a dedicated path for returning unused toner particles to the developing device after collecting the scattered toner particles with the toner collecting roller and scraping the collected toner particles therefrom. This arrangement is disadvantageous in that the provision of the toner returning path results in an increase in machine size. Another disadvantage of this arrangement is that a blade or like means provided for scraping off the collected toner particles from toner collecting roller accelerates deterioration of the toner particles.
What is most problematic in the touchdown development system is a development ghost phenomenon. It is important to scrape off unused toner particles adhering to the development roller by means of the magnetic roller to overcome the ghost phenomenon. As process line speed increases, it is needed to supply an adequate amount of toner particles necessary for developing a larger number of electrostatic latent images to the toner carrying member (development roller) in a short time and, because the period of time available for forming a toner layer decreases, there arises the need to take measures to increase the toner concentration in the two-component developer, for instance. This means that the two-component developer collected and returned to the two-component developer storage space after formation of the toner layer has a higher toner concentration when the process line speed is high compared to a case where the process line speed is low.
Moreover, since the period of time available for scraping off the unused toner particles from the development roller becomes shorter and the toner concentration in the two-component developer collected and returned to the two-component developer storage space becomes higher, it is more difficult to scrape off the unused toner particles from the development roller at increased process line speeds. Additionally, toner scattering is more likely to occur and the scattered toner particles may adhere to the development roller at increased process line speeds, resulting in an increase in the amounts of collected toner particles and scattered toner particles and an increased tendency for the ghost phenomenon to occur due to inadequate removal of the unused toner particles.
Especially in such a high-speed machine based on the touchdown development system with a drum line speed of 180 mm/sec or higher, it is even more difficult to collect the scattered toner particles. For example, a high-speed machine with a drum line speed of 180 mm/sec can print on approximately 40 sheets of A4-size paper per minute in landscape format, those with a drum line speed of 250 mm/sec can print on approximately 50 sheets of A4-size paper per minute in landscape format, and those with a drum line speed of 340 mm/sec can print on approximately 60 sheets of A4-size paper per minute in landscape format.